DESCRIPTION (adapted from the applicant s abstract) Altered function of the neostriatal cholinergic interneurons has been implicated in the pathology of Parkinson's disease, Huntington's disease, and a variety of other disorders. Despite the explosion of information on the pharmacology of acetylcholine in the neostriatum, physiological information has been difficult to obtain due to the relative rarity of cholinergic interneurons compared with the spiny projection cells. Using infrared differential interference contrast microscopy, we have recorded from identified cholinergic neurons in slices, and shown that they are intrinsic pacemakers that fire in three distinctly different spontaneous firing modes in the absence of synaptic input. This finding provides a window on several otherwise inexplicable observations, including the rhythmic synchronous activity of these neurons in monkeys rendered Parkinsonian by experimental treatment with MPTP. In the proposed experiments, we will employ whole cell recording of identified cholinergic interneurons in slices to determine: (1) the ionic conductances that give rise to the three spontaneous firing modes, (2) the way in which the spontaneous firing is controlled and disrupted by afferent and intrinsic synaptic input to the cholinergic cells, and (3) the dynamics of intracellular calcium which we hypothesize acts as a critical intermediary in the synaptic control of cholinergic cell activity. Calcium imaging using intracellular filling with the indicator fura-2 will be used to directly examine the ongoing cyclic fluctuations in intracellular calcium level, the spatial distribution of calcium transients within the cell, and the interaction between synaptic inputs and intracellular calcium dynamics that sculpts the firing pattern. Following physiological recordings, neurons loaded with biocytin will be prepared for subsequent anatomical analyses and these data will be combined and used in computer simulations to investigate the dynamic interaction between synaptic inputs and intrinsic properties in the generation of natural firing patterns.